1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In an electrophotography image forming apparatus, the following method has been known as a method of correcting transfer positional deviations (hereinafter, referred to as “positional deviation” or “color deviation” in some cases) for respective colors, for example. That is, known has been a method of forming a positional deviation correction pattern on an image carrier such as a carriage belt and an intermediate transfer member for conveying a recording medium such as a sheet and detecting positional information of the positional deviation correction pattern formed on the image carrier with a sensor so as to correct the positional deviations based on the detected positional information.
For example, Japanese Patent Application Laid-open No. 2005-031263 discloses the following technique in order to correct a positional deviation normally. That is, disclosed is a technique of using a mode A and a mode B as the situation demands. In the mode A, the positional deviation is corrected by using a positional deviation correction pattern having a normal size. In the mode B, the positional deviation is corrected by using a positional deviation correction pattern having a size and an interval that are larger than those in the mode A such that a sensor detects the positional deviation correction pattern even if the positional deviation is large.
Furthermore, the following method has been already known in order to detect a plurality of positional deviation correction patterns at high speed and with high accuracy. That is, known has been a method of generating interrupt on a central processing unit (CPU) so as to store a detection result (acquire a detection result) in a memory if the predetermined number of positional deviation correction patterns are detected by a sensor.
In the conventional positional deviation correction method using the interrupt on the CPU, an interrupt interval (interval at which the sensor acquires a detection result) until subsequent interrupt is generated from generation of one interrupt depends on a cycle (sampling cycle) in which the predetermined number of pieces of data (A/D-converted data) obtained by converting an analog signal output from the sensor to a digital signal are sampled. Because the sampling cycle relates to various functions such as resolution based on a filter characteristic and a clock, the sampling cycle cannot be changed easily. If an image formation speed is changed in a state where the sampling cycle is constant, the following problem arises in the technique disclosed in Japanese Patent Application Laid-open No. 2005-031263. That is, there arises the problem that the positional deviation correction pattern is not within an expected interrupt interval and cannot be detected normally. In addition, the same problem also arises in a method of correcting densities of respective colors by using a density deviation correction pattern.
There is need to provide an image forming apparatus that can detect an image quality adjustment pattern image normally even if an image formation speed is changed.